In detectors used in a trespass warning system, the system which has been put to frequent use in recent years comprises two pyroelectric elements made of ferroelectric material horizontally disposed side by side, the outputs thereof being connected in series or parallel to derive a differential output. This is called the twin type pyroelectric elements or twin sensors. Further, a compound-eye crime prevention sensor system, which is called the dual twin type pyroelectric elements, comprising two, upper and lower, twin type pyroelectric elements has already been put to practical use. For this compound-eye crime prevention sensor system, refer to Japanese Patent Publication No. 12,358 of 1991 and Japanese Laid-Open Patent Publication No. 143,873 of 1993.
The compound-eye crime prevention sensor system comprises a pair of detection axes substantially horizontally spaced and extending in substantially the same direction in a predetermined space, another pair of detection axes disposed above or below said pair of detection axes and extending in directions respectively corresponding to said detection axes, a far infrared ray focussing mirror disposed on the side of the device associated with these two pairs of detection axes for focussing radiant energy falling along each axis, and dual twin type pyroelectric elements corresponding to them at the focal point corresponding to said two pairs of detection axes in the focal planes of said far infrared ray focussing mirrors.
In said compound-eye system, the four detection zones formed by the optical unit comprising the dual twin type pyroelectric elements consisting of the two, upper and lower, twin type pyroelectric elements and by one far infrared focussing mirror, extend along said detection axes in substantially the same direction but, stated strictly, they are geometrically not parallel (widening angle being 0.degree.); the four detection zones extend with some amount of widening angle. In the present invention, the wording "substantially parallel" or the like also refers to the case where they extend with some amount of widening angle. The size of the upper and lower twin type pyroelectric elements, and the distance between the upper and lower elements have been preset at particular values. The detection zones respectively associated with the detection axes and extending from the optical unit constituted by the twin type pyroelectric elements of fixed shape and the far infrared ray focussing mirror having a fixed focal length grow thicker as they are further away from the optical unit. In the detector for detecting trespassing subjects, with consideration given to the size of the cross section of the detection zones and the size of human beings who walk across the detection zones, the limit distance beyond which the detection efficiency drops is defined as the farthest point of the trespassing subject detection range.
For example, the size of the cross section of one detection zone in the farthest point in this kind of detector for which the distance to the farthest point of the trespassing subject detection range is set at 12 m, is about 0.3 m wide and about 0.5 m high. There are four detection zones of this size disposed at upper and lower and right and left positions, 0.3 m spaced away from each other, and grouped together to form the detection zone of a compound-eye crime prevention sensor system.
FIG. 1 is a side view showing the layout of a conventional compound-eye crime prevention sensor system. The numeral 1 denotes a detector and L denotes the position which determines the farthest point of the trespassing subject detection range. This detection range is defined by upper and lower detection zones Z1, Z2 extending in substantially the same plane from upper and lower twin elements. The numeral 2 denotes a ceiling surface to which the detector is fixed. A human being H moving in a direction from the viewer's side of the drawing to the other side of vise versa in the vicinity of the position indicated by L will cross the upper and lower detection zones at the same time without fail, whereas a small animal N moving about in this place never crosses the upper and lower detection zones at the same time. The detection zones Z1' and Z2' shown in broken lines in the figures define the trespassing subject detection range which results when the optical unit in the detector 1 is turned downward to move the detection zones to the position S close to the detector.
The effective distance between the detection zones Z1' and Z2' decreases by an amount corresponding to the amount by which the distance to the farthest point is shortened in the detection zones subsequent to the change as it moves to the position S. Therefore, a human being H walking in the vicinity of the position S in a direction from this side of the drawing to the other side of vise versa crosses the upper and lower detection zones at the same time, of course. However, when a small animal passes the position S, it moves transversely of both the upper and lower detection zones while nearly grazing these detection zones.
Therefore, in a compound-eye crime prevention sensor system which, only when a subject crosses the upper and lower detection zones at the same time, performs a particular signal treatment to decide that it is a trespassing subject, there is a possibility that the movement of a small animal is erroneously recognized as the movement of a trespassing subject (human being) despite the fact that is actually the movement of a small animal if the spacing between the upper and lower zones is decreased in the detection space by changing the angle of the detection zones extending from the detector. Thus, in the conventional system in which the geometrical relation between the upper and lower detection zones is fixed, there has been a problem that the distinction between the human being and a small animal cannot be clearly made.